Automatic control system



Sept. 1, 193$. p O FARNHAM 2,052,613

I AUTOMATIC CONTROL SYSTEM Filed Jan. 8, 1932 v gmemtok 6/ WM, w

Patented Sept. 1, 1936 STATES AUTOMATIC CONTROL SYSTEM Paul 0. Farnham,Boonton, N. J., assignor, by mesne assignments, to Radio Corporation ofAmerica, New York, N. Y., a corporation of Delaware Application January8, 1932, Serial No. 585,593

v14 Claims. (01. 250-20) Thisinvention relates to automatic controlsystems of the generaltype employed in radio receivers, andmoreparticularly' to a system for automatically preventing noisyoperation of a sensitive receiver. I j

The high amplification obtained when a receiver of the automatic gaincontrol type is tuned to frequencies at which there is no signal, oronly a weak signal, is accompanied by a relatively high noise output'arising in the' tubesand circuits as well as in static disturbances.This high noise output may be quite disturbing in tuning when zonesof'high noise level are passed in going from one signal frequency toanother.

Circuit arrangements are known for providing a sharp cut-off forallreceived signal energy be-.

7 low a predetermined. magnitude, the cut-ofi being obtained by applyingan initial bias to a diode demodulator to suppress all audio responseuntil 1 the signal voltage impressed upon the diode rises above thatvalue which is essential for a satisfactory signal-to-noise ratio.Unfortunately, however, the bias voltage required for bestnoisesuppression is ,sohigh as to produce some distortion in the audioreproduction. H

,An object of this invention is to provide aradio receiver of the typeincludinga noise-suppression system and an automatic gain controlsystem, and in whichth'e automatic gain control system operate'sjtoremove the noise-suppression bias from the demodulator when theradioinput of the desired signal rises above a predetermined value. Morespecifically, an object is to provide a radio receiver includingadetector and an automatic gain control system, the detector-being biasedby the potential drop across a resistance in the output circuit of adirect current amplifier, and in which the flow of current in theresistance, and therefore the bias on the detector, is determined bytherectified voltage developed by, the automatic gain'control system.

These and other objects and advantages of the inventionwill be apparentfrom the following specification when taken with the accompanyingdrawing in which the single view is a circuit diagram of one embodimentof the invention.

'3, of a. radio frequency amplifiertube-4. The amplified output fromtube 4 is passed toa network, indicated generically at5, which maycomprise a simple coupling impedance or may include additional amplifierstages operating at carrier and/or an intermediate frequency.

The input impedance of the diode rectifier is coupled to the outputterminals of the network 5 and, as illustrated, the input impedancecomprises an inductance 6 in series with a tuning con denser '1 andblocking condenser 8, the junction of the condensers being connected toground and to the low potential output terminal of the network 5. Thecoupling to the network 5 may be of any appropriate type, a capacitivecoupling through condenser 9 being illustrated as typical of one form.

The multiple diode rectifier l may be, and preferably is, a commercialtriode of the type having a heater H, an equipotential cathode l2, gridl3 and plate I4. For convenience of description, the electrode [3 willbe referred to as the grid, but it will be apparent from the followingdescription that the grid 13 functions as the anode of a dioderectifier. The high potential side of the tuned input circuit isconnected to the cathode l2 and, so far as concerns carrier irequencies,the input circuit is completed to the rectifier anodes by the radiofrequency by-pass condenser l which is connected between the groundedterminal of the input circuit and the grid, and the audio frequency by-pss condenser l6 which is shunted across the grid and plate terminals.

The tube l0 functions as a double diode having a single input circuitand individual output circuits for audio frequency and for direct cur--rent, Referring first to the direct current output circuit,rectification of the average signal voltage takes place between thecathode I2 and plate I 4, thus developing a direct current potentialacross the resistor I! that is connected between plate 14 and ground,which potential is impressed on the amplifier 4 asa gain-controlvoltage, in the known manner, through an alternating current filtercomprising series resistances l8 and shunt capacities l9. To efiect adelay in the development of direct current potentials as the receivedsignal energy increases to that value which will provide a normal inputvoltage across the diode input circuit, the diode cathode I2 isconnected by a lead to a source of potential which is positive withrespect to the plate I 4.

0n the drawing, this direct current source is indicated by the legend+(40 volts), which negative bias of 40 volts on the anode will postponethe development of a rectified gain control voltage until the peaksignal input on the diode rect current voltage for reducing the gain ofthe amplifier 4.

In accordance with the present invention, the direct current bias on thediode demodulator is varied automatically as a functionof the receivedsignal energy. The audio-frequency voltage developed across resistance2| is applied, through a coupling condenser 22, tothe voltage divider 23which provides means, in known manner, for regulating the audio outputlevel.

The automatic variation of the bias on the grid 13 is provided by.connecting the terminal 24 of the audio output resistor 2i to the plateP of a control tube 25 and, through a resistance 26,.to the lad 29 thatis connected to the direct current source.'

The cathode K of the. control tube is grounded, and the'grid G isconnected to the plate Id of the diode rectifier, preferably through onesection of the alternating current filter l8, l9.' The plate circuit ofthe control tube 25 is shunted by a condenser 'ZTWhich has a lowimpedance for alternatingcurrents.

It will be apparent that, as the carrier voltage 3 E across the dioderectifier input increases to values above the bias voltage applied tothe plate M, the direct current potential applied to the grid G of thecontrol tube becomes more negative and the plate current flow throughthe output resistance 26 of the control tube is reduced'. 'When thedirect current potential on gridG-blocks the flow of plate currentthrough the control tube output resistance 26, the direct currentpotential on the grid I3 of the diode demodulator is-the same as that ofits cathode lz. The demodulator therefore functions as the usualunbiased diode and the audio response is a substantially undistortedreproduction of the modulation envelope of the signal energy impressedon the diode;

When the received signal energy falls to such lowilevels that theamplified signal voltage E0 at the input circuit of tube i0 is below thevalue effective to causerectificationbetween plate M 'and'cathode 12,the grid G and cathode K of the-control tube or direct current amplifier25 are at the same potential and a flow of plate current through theresistance 26 results. The potential drop due to this plate current fiowreduces the direct current potential of plate P and therefore of theterminal 24 of the audio output resistance 2|, thus making the potentialof the demodulator anode, i. e.,' the grid I13, more negative than thatof the cathode i2. This negative bias potential suppresses the audiooutput of the diode demodulator and, until the input voltage E0 rises tothat substantial value which provides a rectified voltage on the grid ofthe control tube 25, or which exceeds the negative bias on thedemodulator anode M, there will .be no development of. an-audiofrequency voltage across the resistances and the: voltage divider 23.-This postponement of all audio output'until the received signal energyreaches a,

predetermined value effectively prevents noisy operation or weak signalsbut, since the delay'bias is automatically reduced or removed as thesignal strength increases, there is no distortion in the reproduction ofsignals 'of substantial magnitude.

While I have described one particular embodi ment which has provedsatisfactory, itwill be apparent that there is some latitude in thechoice and the relative arrangement of the circuit elements whichprovide anautomatic variation in the delay bias voltage impressed uponthe demodulator.

I claim: 7

1. The combination with a radio amplifier, and an audio transmissionsystem including a demodulator, of directcurrent amplifier means opera-.

tive to block transmission through said audio transmission system, meansincluding a rectifier for automatically controlling the gain of saidradio amplifier in accordance with variations in the strength of areceived signal, and means including said rectifier for rendering saidblocking means inoperative when the input voltage on said rectifierexceeds'a predetermined value. 7 V

2. In an electrical wave transmission system, the combination with anamplifier, a demodulator, means for controlling the gain of saidamplifier, said means including a rectifier developing a direct currentpotential which varies with the strength of a received signal, theelectrodes of the demodulator and rectifier being disposed in a commontube envelope and means normally block- 1ng operation of saiddemodulator, of means actuate d by the direct'cu'rrent potentialdeveloped by said rectifier for reducing the efiectfof said blockingmeans.

I 3; In a radio receiver, the combination of a radio amplifier, a diodedemodulator, direct cur-' rent amplifier biasing means impressing a biasvoltage on said demodulator to block operation thereof, means includinga rectifier for automatically controlling the gain of said amplifier inac cordance with variations inthe strength of are ceived signal, andmeans including said rectifier for automatically varying the magnitudeof the bias voltage impressed on said demodulator by said blockingmeans.

4. In a modulated carrier wave receiver, the

combination with an amplifier, a demodulator, and means including arectifier for automatically varying the gain of said amplifier when there-. ceived signal energy exceeds a predetermined low level, of directcurrent amplifier means biasing said demodulator to prevent thedevelopment of an audio frequency output, and means controlled by thesaid rectifier for neutralizing said biasing means when the receivedsignal energy exceeds a predetermined value.- r

5. A combined demodulator and carrier volt age rectifier, comprising adouble diode rectifier having a cathode and a pair of anodes, an inputcircuit -resonant at carrier frequency and connected between therectifier cathode and ground, a direct current output circuit connectedbetween one anodeand ground, an a1 ternating current output circuitconnectedbetween the second anode and apoint of negative potential withrespect to said cathode, and means for reducing the magnitude of thenegative potential of said point; said means including a control tubehaving the grid thereof subjected to the potential developed by saidoutput circuit, anda plate circuit for said control tube includingareeluding a resistance in the circuit of said detector for applyingthereto a delay bias voltage to render the detector inoperative forradio input voltages below a critical value, of means including saidrectifier and a direct current amplifier for establishing a current flowin said resistance to reduce said applied bias voltage when the radioinput voltage on said rectifier exceeds a predetermined value, and meansenergized by said rectifier for automatically controlling the gain ofsaid amplifier in accordance with variations in the strength of theradio frequency voltage impressed on said receiver.

7. In a radio receiver, a radio amplifier, a demodulator diode, meansincluding a second diode for automatically controlling the gain of saidamplifier in accordance with variations in the strength of receivedsignals, and biasing means including one of said diodes for impressingupon said demodulator diode a bias voltage that varies with themagnitude of received signals in that sense which renders saiddemodulator diode inoperative for weak signals and operative for signalsin excess of a threshold value.

8. In the reception of radio signals by a receiver including a radioamplifier, a detector diode and an automatic gain control systemincluding a diode; the method which comprises initially impressing abias voltage on one of said diodes to render the same inoperative forreceiver input voltages falling within a predetermined and relativelylow range of values, and automatically reducing the bias voltage on saidbiased diode when the receiver input voltage exceeds said range ofvalues.

9. In the operation of a radio receiver including a pair of diodedetector elements, a pair of diode elements forming part of an automaticgain control system, an amplifier including a cathode and a plate, and aresistance in the platecathode circuit of said amplifier, the method ofcontrolling the operation of one of said pairs of diode elements whichcomprises impressing between the same as a bias voltage the directcurrent potential drop produced across the resistance by plate currentflow in the amplifier, and automatically varying the flow of platecurrent as a function of the radio input to the receiver, thereby tovary the bias on the controlled pair of diode elements.

10. In the operation of a radio receiver having a radio amplifier and apair of diodes comprising elements in a demodulator and in an automaticgain control system, the method of control which comprises biasing onediode to render the same inoperative for receiver input voltages of lessthan a predetermined value, producing a direct current voltage by therectifier action of the second diode, and employing the said directcurrent voltage to reduce the bias on the first diode.

11. In a radio receiver, the combination with a radio frequencyamplifier and a detector, of means responsive to the magnitude of theradio voltage input to said amplifier for biasing said detector torender the same inoperative for input voltages below a predeterminedlevel and for reducing the applied bias voltage when the input voltageexceeds said level, said means including a direct current amplifierhaving an output resistance comprising a biasing impedance for saiddetector.

12. In a radio receiver, the combination with a radio frequencyamplifier, a detector diode, and a diode included in a system forautomatically controlling the gain of said amplifier, of meansresponsive to the magnitude of the radio voltage impressed on saidreceiver for biasing one of said diodes to render the same inoperativefor input voltages below a predetermined level and for reducing theapplied bias voltage when the input voltage exceeds said level, saidmeans including a direct current amplifier having an output resistancecomprising a biasing impedance for said detector.

13. In an amplifier, means for deriving a direct current proportional toan incoming signal, a space current device including an anode and acathode, means for normally maintaining said anode negative with respectto said cathode, and means whereby the said direct current is utilizedto cause a reversal of the polarity of said electrodes, with consequentflow of space current.

14. In a modulated carrier wave receiver, the combination with a carrierwave amplifier and an audio transmission system, means including a dioderectifier for automatically varying the gain of the amplifier when thereceived signal energy exceeds a predetermined low level, means,including an electron discharge tube, operative to block transmissionthrough said audio system, an input electrode of said last tube beingconnected to said diode rectifier for rendering theblocking tubeinoperative when the signal input voltage on said rectifier exceeds apredetermined value.

PAUL O. FARNHAM.

